With fast pace of urbanization and well road construction, road lighting is playing an important role in the whole city lighting system. When developing a lighting device for road lighting, different requirements have to be satisfied. First, the most important target for road lighting is to ensure road traffic safety, and to achieve this goal, the factors such as the road surface luminance, no glare, the visibility of objects, etc., should be considered. However, among these factors, the visibility of objects is always neglected for road lighting design.
Second, the energy saving of the lighting device should be taken into account. To reduce energy consumption, some intelligent lighting control systems take an energy-saving approach by reducing the power of road lighting at midnight with a sacrifice of average luminance level. Such an approach is based on the assumption that at midnight, the vehicle flow rate is relatively lower, so the visual tasks for drivers are easier, and a lower luminance level can be still sufficient. However, at midnight, drivers tend to be tired and drowsy; therefore reducing the lighting level could lead to a risk of traffic accident.